Hi, Spiceyben,
I don't have a 380p vid to test, but I would assume that a 380p vid would work in iMovie. Do you have enough free space on your application disk for iMovie to operate efficiently? Is perhaps the format of the vids one that is causing iMovie to struggle? Do they import and play correctly when imported to your hard drive outside of iMovie?
Try creating a new project and importing a 380p clip into it as the first clip. That would set the project at 380p. See if it imports.
Also you could try deleting/resetting preferences. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie usually will
open in a new empty library. If so, reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Another procedure would be to open iMovie in a new library. Do that by opening iMovie while holding down the Option key and selecting to create a new library. IMovie will open in that library. It will be blank, so create a project in it and try to import one of the 380p vids.
It also might work to try converting the clips with the free download, Handbrake, to Mp4/AAC that Handbrake will do by default. You would first need to import the clips to your desktop. You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to
complete. Then save and import the converted clip into iMovie.
-- Rich